Pesticide Resistance in L. Salmonis
The parasite Lepeophtheirus salmonis ''(also known as the sea louse) feeds on the blood of ocean salmon. ''L. salmonis ''is a notorious ravager of densely-populated marine cages where salmon are farmed. According to a 2009 estimate, ''L. salmonis ''is responsible for $380 million in losses worldwide each year. To combat these losses, most salmon farmers treat their fish with emamectin benzoate, a neurotransmission inhibitor that is lethal to ''L. salmonis. ''Unfortunately, ''L. salmonis ''has rapidly developed a P-glycoprotein efflux pump that can force emamectin benzoate out of its cells. To further understand how this pesticide resistance developed as well as how it spread throughout the Atlantic ocean, a genomic analysis study of ''L. salmonis ''DNA was conducted. Single Nucleotide Polymorphism Analysis for Pesticide Resistance Researchers at Norway's Institute for Marine Research developed an array that detected 6000 SNPs. 5000 of these SNPs were distributed throughout the genome every 100 bp, with an additional 200 every 10k bp. Another 600 SNP's were screened for as indicators to distinguish the geographic origin of the sample. Approximately 200 SNP's were clustered around the gene suspected to be involved in emamectin resistance by expressed sequence tags. 576 ''L. salmonis ''genomes from twelve salmon farms across the North Atlantic were sequenced and put through the SNP array. The researchers found that there were no genomic differences between ''L. salmonis ''from each of the different farms, suggesting that ''L. salmonis ''populations throughout the Atlantic ocean are genetically very similar. This corroborates other genomic studies on ''L salmonis ''that found highly conserved mtDNA and microsatellite regions across populations, observations concurrent with the fact that salmon from around the world often carry ''L salmonis ''to common ocean feeding grounds where they can mix. Furthermore, these genomic similarities imply that such a large population of organisms would not be highly susceptible to genetic drift, ruling out the notion that emamectin resistance developed only by random mutation. Instead, it is more likely that emamectin resistance originated in one location before spreading to other regions. The researchers also detected a very high frequency (~10%) of a well-conserved haplotype in the region for emamectin resistance in all 12 farms. These genes were subjected to BLAST alignment and found to potentially play a role in emamectin resistance. To confirm that these genes were in fact the genes of emamectin resistance, a genomic-wide association study was performed. ]] The researchers took another 576 ''L. salmonis ''individuals and sorted them at into two groups, one group of known emamectin resistant individuals and unknown resistance individuals, and another group of only unknown resitance. Multiple generations of offspring of these two groups were exposed to emamectin benzoate to demonstrate the selective sweep favoring offspring with emamectin resistance genes. As shown in the Manhattan plots at the right, there was a significant decrease in the variance of these genes, indicating that emamectin resistant ''L. salmonis ''were selected for. This confirms the link between the selective sweep that was observed in the GWAS study and pesticide resistance. References Besnier F, et al. 2014. [http://www.biomedcentral.com/1471-2164/15/937/abstract "Human-induced evolution caught in action: SNP-array reveals rapid amphi-atlantic spread of pesticide resistance in the salmon ectoparasite ''Lepeoptheirus salmonis."] ''BMC Genomics. ''15:937. Igboeli OO, et al. 2012. "Role of P-glycoprotein in emamectin benzoate resistance in sea lice." ''Aquaculture. ''